In anaesthesia, anaesthetic gases are delivered to subjects, such as patients, usually utilizing one of a number of types of anaesthetic circuit. The anaesthetic circuit is arranged to deliver a regulated dose of anaesthetic to the subject along with oxygen or a gas mixture including oxygen or atmospheric air. It is important that the subject be supplied with and breathes a correct mixture of oxygen and anaesthetic gas. Too little oxygen reaching the lungs, or too much re-breathed CO2 reaching the lungs, can lead to significant problems, including hypoxia and death.
An important issue with anaesthetic circuits is the issue of “dead space”. This is the volume of gas at expiration that the subject must re-breathe before getting gas that does not contain carbon dioxide. The dead space includes the subject's own air passageway dead space (e.g. trachea, or cavity) and, with anaesthetic circuits, a certain amount of “machine dead space”.
The more machine dead space the more volume a subject must breathe in before they breathe gas that does not contain carbon dioxide. For adult humans and large animals, the amount of machine dead space is often not large compared to the anatomic dead space and the total volume of their airways. For small animals and small humans, however, the amount of machine dead space can become critical. For example, a two kilogram animal may have a total tidal volume of 20-30 mls. A third of this volume will be anatomic dead space. Any additional machine dead space may cause significant issues for anaesthesia. For example, an extra 10 mls of dead space can cause the animal to hyperventilate, become hypoxic and die.
Even if patients do not become hypoxic, the delivery of anaesthetic is much less efficient and it is more difficult to achieve anaesthesia.
To address the machine dead space issue, it is common to intubate patients (introducing a tube into the trachea). For small animals and infants, however, this is not always a satisfactory solution. With smaller airways, the diameter of the tube lumen becomes so small that the patient may not be able to draw enough gas into their lungs (the small diameter tubing provides significant resistance to inspiration).
Small, tight fitting masks have been designed for small animals and infants. Nevertheless, even small masks still add dead space which can be significant.
In fact, dead space issues for small animals such as lab animals (e.g. rats, mice and other lab animals) are so significant that apparatus and methods other than traditional delivery of anaesthetic gas mixture via masks or tubes must be contemplated. With slow acting anaesthetics, such as ether, simple systems that merely provide the anaesthetic vapour or liquid direct to the patient, e.g. ether from cotton wool balls onto the nose of the patient may be used. Such traditional anaesthetics, however, are environmentally unsound and have generally fallen out of use.
Perhaps the most commonly used systems in the prior art for small lab animals are open systems which actively pump anaesthetic and waste gas over the respiratory openings of the subject. The patient is placed with their respiratory orifice next to a large opening of a delivery system. The anaesthetic is essentially pumped to the subject's respiratory orifice. Waste gas is sucked out of the system by pumping. That is, the waste gas is actively removed by pumping. These systems are high flow systems. Modern anaesthetics such as methyoxyflurane can be used in these systems, but large amounts of anaesthetic, in the order of a liter per minute in some cases, are used. This is very expensive. Also, a significant amount of anaesthetic may leak into the surrounding environment from the open system, causing a potential health hazard. The active provision and removal of anaesthetic/gas mixture is therefore very wasteful and expensive.
With these systems, it is also very difficult to ensure that a subject is anesthetized with any precision. It is difficult to tell with any accuracy the concentrations of anaesthetic gas mixture at the patient's respiratory orifice. There will be, for example, entrainment of room air because of the open system, which will dilute the anaesthetic gas mixture. It can therefore be very difficult to anesthetize subjects with this system and maintain anaesthetic depth.
It is often required that a number of subjects be anesthetized at the same time. It is common, for example, where a number of lab animals are being treated at the same time. They may be being anesthetized, for example, in order to have blood samples taken so that lab experiments can be monitored, or they may be being anesthetized for other reasons. It is known to anesthetize a plurality of animals in an induction chamber. Again, however, a lot of anaesthetic is used to fill the induction chamber sufficiently to anesthetize the animals, and little or no precision in the amount of anaesthetic administered is possible. This can lead to accidents, not maintaining anaesthesia, or even death of the subjects.
It is known to use active systems such as discussed above, to anesthetize a plurality of animals at the same time. This essentially means reproducing the open system for each subject to be anesthetized. A separate open mask is provided for each subject and a separate line from the anaesthetic machine for each subject is required. This is complex, difficult to set up, and difficult to ensure that the animals are placed appropriately by each respective mask and maintained there while anaesthetic takes effect. The number of lines and flow meters required for the anaesthetic machine is also complex and expensive.
The present Applicant's earlier patent application, International Patent Application No. PCT/AU2007/001920, discloses a method and apparatus for delivering anaesthetic to a patient which delivers the anaesthetic by inducing an anaesthetic flow to reduce re-breathing of fluid exhausted from the respiratory tract of the subject. The exhaled fluid is exhausted from a position distal to the proximal position to the respiratory tract of introduction fresh fluid to the subject. The contents of this application are incorporated herein by reference.